Aftertreatment devices are commonly utilized in order to treat exhaust exiting engines in order to minimize noxious properties of gases exhausted from diesel engines, in particular. A number of technically distinct technologies are typically utilized in connection with the reduction of emissions of oxides of nitrogen (NOx) and particulate patter (PM, or soot) within exhaust.
One method of aftertreatment of exhaust gases, used especially in the road transport sector, is known as selective catalytic reduction (SCR). This uses a catalytic converter in which a water-DEF (“diesel exhaust fluid”) solution is injected into the exhaust gases to reduce NOx emissions before expulsion into the atmosphere.
Yet another aftertreatment method is a lean NOx trap (LNT), which is a discontinuously operating aftertreatment technology. LNT has a number of operating modes including NOx storage during lean engine operation, NOx reduction during rich operation phases, and LNT desulfurization under rich conditions and high temperatures.
A method of NOx reduction is exhaust gas recirculation (EGR) in connection with exhaust gases prior to reaching the aftertreatment devices identified above. In this process, a portion of the exhaust gases is cooled and dosed back into the cylinders of the engine. Before reaching the atmosphere, the engine exhaust gases are cleansed of harmful partially or wholly unburned substances such as fuel or oil in the diesel oxidation catalyst (DOC), without the addition of a DEF solution, such as the SCR discussed above. A diesel particulate filter (DPF) is often utilized to intercept the particulate matter.
In order to maintain efficient operation of aftertreatment devices, the equipment should be cleaned at proper intervals generally based upon mileage, hours of operation, direct or indirect readings, or combinations of the same. Various methods exist for cleaning aftertreatment equipment, depending upon the piece of equipment. Equipment may be removed from a machine for cleaning, for example. Such removal of equipment for frequent cleaning, however, may be undesirable due to costly downtime, particularly when machines are used in the field. Some machines may include supplemental on-board equipment to facilitate cleaning or methods of regeneration. For example, sufficient heat may be provided to oxidize and capture soot accumulated on a diesel particulate filter. Levels of heat required in the regeneration processes of aftertreatment devices, as well as the intervals between such regeneration, may vary.
U.S. Pat. No. 7,808,511 to Bolton et al. controls the richness of the air/fuel mixture entering the engine, or engine air/fuel ratio, during a lean NOx trap (LNT) regeneration process by recirculating some air from the compressor discharge back to the compressor inlet. Bolton utilizes this same method to increase the engine exhaust temperature to promote regeneration of a diesel particulate filter, or increase catalyst light off. Controlling the exhaust temperature by way of this diversion of air from the compressor discharge back to the compressor inlet to control the richness of the fuel entering the engine, however, may result in an undesirable increase in NOx emissions.